I can almost make out his expression. If I wait a second longer, I'll know from the look on his face whether he's a threat or not. But it's a second I don't have. So I pull the trigger. And he falls the same way, every time I think about it.

Looking back, I suppose it's more likely than not he was just trying to get away from the shooting. But I'll never know.

This bothers me.

It's not a moment that defines me. I don't let it. But there's no doubt it shaped me. It taught me what I was capable of doing to other people.

Who understands this sort of thing? Does the public that sent me to Iraq get it? I'm not sure, but when I think about how to explain what it's like, I always go back to fiction.

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There is a moment, early in the series of Breaking Bad, when loser chemist Walter White decides he doesn't possess the brutality to be a criminal. A drug dealer named Krazy-8 tries to kill Walter and his fellow meth-entrepreneur, Jesse Pinkman, and now Krazy-8 is locked in a basement after inhaling Walter's toxic chemicals. The deal is simple: Jesse must dispose of another body, and Walter with Krazy-8.

His written checklist of why to let Krazy-8 live is extensive (It's the moral thing to do); on the side labeled "kill him," there is only one item (He'll kill your entire family if you let him go). Walter searches for any reason not to kill him. He learns about his childhood and his interest in music. They share a memory of a cheesy jingle written for a furniture store Krazy-8 worked at as a kid. Exhausted at the prospect of murder, Walter sets to retrieve the key to free Krazy-8. "This line of work doesn't suit you," he tells Walter.

But as fans of Breaking Bad know by now, with the second half of the fifth and final season premiering Sunday night, there are no morally uncontaminated solutions in creator Vince Gilligan's world. Upstairs, Walter realizes Krazy-8 grabbed a shard of broken plate following Walter's wheezy collapse in the basement, which makes his decision very clear: He must preemptively murder his captive.

Walter, along with several of the Breaking Bad characters, exhibits a term many of us in the military and veterans community have come to understand as a moral injury, and the show profoundly explores the concept in a way previously unseen in film and television. Of course, virtually no troops or veterans have much in common with the criminals in the show, but the reaction to traumatic events is universal, be it in war or a fictional universe.

A moral injury is not a psychiatric diagnosis. Rather, it's an existential disintegration of how the world should or is expected to work.

To be clear, a moral injury is not a psychiatric diagnosis. Rather, it's an existential disintegration of how the world should or is expected to work—a compromise of the conscience when one is butted against an action (or inaction) that violates an internalized moral code. It's different from post-traumatic stress disorder, the symptoms of which occur as a result of traumatic events. When a soldier at a checkpoint shoots at a car that doesn't stop and kills innocents, or when Walter White allows Jesse's troublesome addict girlfriend to die of an overdose to win him back as a partner, longstanding moral beliefs are disrupted, and an injury on the conscience occurs.

As he chokes the life from Krazy-8 with a bike lock, Walter enters a distorted moral universe where killing and death become the currency of his trade. Yet Krazy-8's death haunts Walter throughout the series, evident when he delicately slices the crust of sandwiches he makes for himself just as Krazy-8 requested—a twisted homage. The episode is bookended by a flashback discussion of the soul. What are its scientific properties? Is it even real? Walter's brainy ex-flame Gretchen contends that it coexists with the body. As Walter and Jesse scoop the dissolved, goopy remains of Krazy-8 into a toilet, they are confronted with how the destruction of a soul affects their own.

"The soul?" Walter asks Gretchen in a more innocent time. "There's nothing but chemistry here."

Most Americans are exposed to war on celluloid and pixels, where war boils down to a clearly demarcated struggle between two forces. In many war films, combatants are easily parsed from civilians, and killing becomes a competent exercise of pulling the trigger. The act of killing has been demystified to the extent that the power of its moral decay is seldom seen, rarely felt, and never absorbed. The public appreciates its troops (shallowly, some argue), but it doesn't know them, and as a result, doesn't understand what happened over there—or why it matters years and decades later as war veterans unpack the choices they made.

In Season Five, Jesse's morals are viscerally assaulted when a member of his crew kills a witness to a heist: a young boy. The unassailable innocence of children resonates particularly with moral injury in fiction and reality. I certainly never considered how children could be threatened in war before I deployed to Iraq. A diet of war films and books like Band of Brothers contributed to a conclusion that an American war waged was a war justified by our virtue to do good, and that meant one thing in particular: Only enemy soldiers died by our hands.

An ambush launched from a school during the Iraq surge in restive Baquba vaporized any notions of moral clarity with a bomb blast. A driver in my platoon was killed, and his squad tumbled out the back—dusty, injured, and shaken. Heavy guns tore through the school's compound as insurgents launched rockets and raked us with machine gun fire. There was nowhere for the children to go except their classrooms behind cinderblock walls. Several lay dead and dying in crimson pools as we cleared the building. We evacuated those still alive and continued on our mission to flush out more guerillas. I didn't think much about those kids until much later, when the din of war subsided, replaced with the constant moral buzz of questions: Why did that happen? Was our protection worth their lives? What does my humanity mean now?

After I pulled the trigger, he dropped to the ground and tumbled out of view. I have no idea if he lived or not, but I can now measure my own guilt far more than his.

Walter loses his humanity the more he becomes Heisenberg, his drug kingpin nom de guerre. The diminishment of his moral core is in stark contrast to his brother-in-law Hank Schrader, a foul-mouthed but genuine DEA agent on the trail of Heisenberg's trademark blue meth. Hank's identity rests with his sleuth talents and good-guy persona, but his sense of morality begins to crumble in the face of trauma. Saddled with post-traumatic stress after he kills Albuquerque drug lord Tuco Salamanca, he narrowly escapes an insurgent-inspired IED attack in Juarez.

The encounters with violence take a toll in his career. The good cop finds himself in a seedy bar to provoke a fight with hulky bikers. His partner Gomez watches him closely after. "Do you need to talk to someone?" he affectionately asks. Hank stares him down. "Take your hand off my shoulder," he snipes back.

Hank's hyper-vigilance and outbursts following the attack lead to a violent confrontation with Jesse and consequently his suspension from the DEA. "I've been... unraveling," he quietly admits to his wife, Marie, ahead of his meeting with DEA investigators. "What I did to Pinkman, that's not who I'm supposed to be. That's not me." For the first time, Hank admits to compromising his morals.

Hank is disrupted on his way to redemption after a deadly gun battle with cartel hitmen leaves him paralyzed and isolated from his career case. Marie nudges him toward physical rehabilitation and good spirits, and rebukes from Hank are a departure from his supportive husband role earlier in the series. A break in the murder case of Walter's lab assistant allows Hank to return to his feet, and more importantly, to reassert his identity as a DEA agent. The virtues of the job shift his moral alignment back to the center as he gets closer to Heisenberg. He gets much nearer than he anticipated in the mid-season finale when he discovers Walter's lurid secret.

I see myself in Hank's fragile moments, and his agony over a decision that only later untangled as an existential question of moral integrity. I shot that running man in the hip and stomach even though he didn't have a gun. He was sprinting toward my unit's position following an ambush that sheared one soldier's leg off. After I pulled the trigger, he dropped to the ground and tumbled out of view. I have no idea if he lived or not, but I can now measure my own guilt far more than his.

Soon after, a buddy of mine and I launched grenades into a house presumably filled with insurgents preparing a counterattack. Iraqi police later said more than a dozen of them were killed inside. None of us knew for sure if they were guerrillas or civilians cowering as shrapnel burrowed into walls and bone. A Hellfire missile later collapsed the roof.

Those images come to mind when I get that grim question, "Did you ever kill anyone?" I don't usually entertain it, but it would be a relief to have the finality of an answer, so I can learn to live with decisions I made in an instant that either changed lives or ended them. The real answer, though, is a scab on a moral wound that I pick at every now and then:

Alex Horton is a freelance writer on veterans and foreign affairs. His work has been published by The New York Times, Foreign Policy, and Washingtonian magazine. He served as an Army infantryman for 15 months in Iraq.